A magnetic recording read channel converts an analog read channel into an estimate of the user data recorded on a magnetic medium. Read heads and magnetic media introduce noise and other distortions into the read signal. As the information densities in magnetic recording increase, the intersymbol interference (ISI) becomes more severe as well. In read channel chips, a Viterbi detector is typically used to detect the read data bits in the presence of intersymbol interference and noise.
The Soft-Output Viterbi Algorithm (SOVA) is a well known technique for generating soft decisions inside a Viterbi detector. A soft decision provides a detected bit with a corresponding reliability. These soft decisions can be used by an outer detector to improve the error rate performance of the overall system. For a more detailed discussion of SOVA detectors, see, for example, J. Hagenauer and P. Hoeher, “A Viterbi Algorithm with Soft-decision Outputs and its Applications,” IEEE Global Telecommunications Conference (GLOBECOM), vol. 3, 1680-1686 (November 1989). SOVA architectures exist for one-step trellises, where one soft decision is generated per clock cycle. SOVA detectors may be implemented, for example, in next-generation read channel systems, and data rates in excess of 2 Gigabits-per-second will have to be achieved. It is challenging to achieve such high data rates with existing SOVA architectures that consider one-step trellises.
A need therefore exists for a method and apparatus for performing SOVA detection at the high data rates that are required, for example, by evolving high-end storage applications. A further need exists for a method and apparatus for performing SOVA detection employing a multiple-step trellis.